Various types of hitch assembly control systems for work vehicles such as agricultural tractors have been proposed, designed, and used. Among the main objective of such systems is the need to regulate the elevation of the hitch with respect to the vehicle main frame, which also serves to regulate the position of the implement (e.g., a plow) attached to the hitch assembly during operation. Typically, such hitch assembly control systems have been operable in any of a number of different modes (also referred to as states). For example, the hitch assembly control system shown in U.S. Pat. No. 5,549,166 is operable in any of the following modes: an enable mode; a position mode; a draft mode; a hitch up mode; and a momentary actuation mode. The operation of the hitch assembly control system in these modes is described in the '166 patent, and also in the paper entitled "Electro-Hydraulic Draft Control (EDC) System Description for a Variety of Tractors", SAE-941764, by Abraham Orbach.
The two states of operation most pertinent to the present application are the position state and the draft state. In the position state, the hitch objective is set by the position control lever, and the hitch assembly is commanded to follow the commanded lever position, except that hitch upward motion is limited by the setting of a height control knob. The position state can exist whether the tractor is moving or stationary, and the position state is the basic hitch control for the tractor in that it is not an option. In the draft state, the hitch assembly is commanded to maintain a substantially constant draft on the tractor, and toward this end the hitch assembly is allowed to move elevationally within predetermined limits. The draft state can only exist when the tractor is moving and the tractor is equipped with appropriate draft sensors. Transition into the draft state can be accomplished from the position state.
The logic under which the hitch assembly control system of the '166 patent transitions from the position mode to the draft mode is shown in FIG. 11B of that patent. As shown, the system transitioned from the position mode to draft mode when the UP/DOWN switch was down, the tractor was moving, the remote switches were off, there was no DEGRADED2 fault (i.e., the draft force command knob and at least one of two draft force sensors were properly working), the draft option was installed on the tractor, and the sensed draft load was greater than the commanded draft load for more than a predetermined time period (e.g., 0.1 seconds). If these conditions were all true, then a transition from position to draft mode occurred.
The logic in FIG. 11B of the '166 patent was later modified to add an additional check before allowing the transition from position to draft mode. This added check, between steps 732 and 738 on the flowchart of FIG. 11B, verified that a desired or "target" hitch position was greater than the hitch position command before allowing the transition to draft mode, with the target hitch position determined based upon the settings of the load command knob and travel knob in the same manner as in draft mode, as described in the '166 patent and the "Electro-Hydraulic Draft Control (EDC) System Description for a Variety of Tractors" paper referenced above.
This added check, however, has proven inadequate due to the fact that the actual hitch position can lag behind the hitch position command. For example, if the operator raises the hitch on exiting a row using the position command knob, and then lowers the hitch using the position command knob upon re-entering the next row in the field, the actual hitch position will lag behind the hitch position command. Depending upon the drop rate, the hitch may take up to 12 seconds or so to be lowered all the way down to the commanded hitch position. In this case, the hitch position command will be below the target hitch position, but the actual hitch position will be above the target hitch position due to the time required to lower the hitch. Thus, despite the added check, the control system may still prematurely enter the draft mode even with the target hitch position less than the actual hitch position.
Accordingly, it would be desirable to modify the transition logic which governs the transition from the position mode to the draft mode to account for the lag between the actual hitch position and the hitch position command. It would also be desirable to modify the transition logic to prevent transition from the position mode into the draft mode when the target hitch position is below the actual hitch position.